I am running into a problem reproducing experiment 15 in the book Make Electronics by Charles Platt.
The top part of the circuit work well (the siren part) but for some reason, the relay does not engage when I disconnect the magnetic contacts.
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Hard for me to know where your problem is. Your circuit is very neatly built, and I doubt you have made any wiring errors. The first step is to attach your black meter probe to negative ground (perhaps with a double-ended alligator test lead) and use the other probe to find out if a voltage is being supplied to the relay, and if so, what voltage it is. If you are getting at least 11V to the relay coil, I have to wonder if there is something wrong with the relay, or if you are supplying voltage to the wrong row of holes on the breadboard.